The present invention relates to a copying lens system, and more particularly, to a copying lens system that has a brightness of approximately F.sub.NO 1:6.5 -8, that covers a wide field of up to approximately 25-30 degrees in terms of half-view angle (.omega.), and that is capable of copying over a magnification range of 0.64 (reduction) through 1x (life size) to 1.42 (enlargement).
Increased demand for reducing the size and cost of copying machines has required, correspondingly, the designing of smaller and less costly copying lens systems. A copying lens system known in the art and used for this purpose is a so-called "split dagor" type lens that has a symmetrical six-element composition, and which covers a wide field of up to approximately 25-30 degrees in terms of half-view angle (.omega.). This type of lens is described in several patents such as JP-B-49-28029 (the term "JP-B" as used herein means an "examined Japanese patent publication") and (JP-A-51-120723 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Known examples of a lens system having a symmetrical four-element composition that is compact and manufactured inexpensively are described in U.S. Pat. No. 2,865,252, JP-A-59-45418, etc.
With the recent advances in electrophotography technology, photoreceptors having an increased sensitivity have been developed, and the use of copying lenses having a brightness in the range of F.sub.NO 1:7-8 has become popular. A split dagor type lens system composed of six lens elements symmetrically arranged is capable of providing brightness in the range of approximately F.sub.NO 1:4.5 to 1:5.6. However, this range is insufficient to satisfy fully the need for size and cost reduction of recent copier models that employ a lens system having a brightness in the range of F.sub.NO 1:7-8.
The lens system of a symmetrical four-element composition described in U.S. Pat. No. 2,865,252 covers a wide field of up to approximately 30 degrees in terms of half-view angle, but this suffers from the disadvantage of not only low brightness (ca. F.sub.NO 1:10), but also high machining cost due to the small radius of curvature compared to the focal length. The lens system described in JP-A-59-45418 is improved in brightness (ca. F.sub.NO 1:8), but field coverage is only capable of covering a half-view angle of up to approximately 20 degrees.